Preparation of polyethylene waxes

ABSTRACT

In a process for preparing polyethylene waxes at from 200 to 350° C. and pressures in the range from 500 to 4 000 bar using molar mass regulators, a peroxide mixture comprising from 5 to 95% by weight of at least one cyclic peroxide of the formula I,                    
     where the radicals R are identical or different and are selected from among alkyl groups and aryl groups, is used as free radical initiator and a molar H 2 /ethylene ratio of from 1:2 000 to 1:40 000 is employed.

The present invention relates to a process for preparing polyethylenewaxes at from 200 to 350° C. and pressures in the range from 500 to 4000 bar using molar mass regulators, wherein a peroxide mixturecomprising from 5 to 95% by weight of at least one cyclic peroxide ofthe formula I,

where the radicals R are identical or different and are selected fromamong alkyl groups and aryl groups, is used as free radical initiatorand a molar H₂/ethylene ratio of from 1:2 000 to 1:40 000 is employed.

The preparation of homopolymers and copolymers of ethylene byhigh-pressure processes is carried out industrially on a large scale. Inthese processes, pressures above 500 bar and temperatures of 150° C. andabove are used. The process is generally carried out in high-pressureautoclaves or in tube reactors. High-pressure autoclaves are known insquat or elongated embodiments. The known tube reactors (UllmannsEncyclopädie der technischen Chemie, Volume 19, p. 169 and p. 173 ff,(1980), Verlag Chemie Weinheim, Deerfield Beach, Basle, and Ullmann'sEncyclopädie der technischen Chemie, 4th Edition, keywords: waxes, Vol.24, p. 36 ff., Thieme Verlag Stuttgart, 1977) are easy to handle andhave low maintenance requirements and are advantageous compared tostirred autoclaves. However, the conversions which can be achieved inthe abovementioned apparatuses are limited and generally do not exceed30%.

To increase the capacity of available apparatuses, attempts are made toachieve very high conversions. However, limitations are imposed bypolymerization temperature and polymerization pressure which, dependingon the product type, have a specific upper limit. For LDPE waxes, thisupper limit is about 330° C.; above this, spontaneous ethylenedecomposition can occur. Furthermore, efforts are made to improve heatremoval by means of a very low wall temperature. However, below atemperature of 150° C., heat removal problems can occur as a result ofthe formation of laminar polyethylene layers which can act as insulator.Furthermore, the pressure drop which occurs is a limiting factor; thispressure drop increases with decreasing temperature.

The conversion can be increased within certain limits by appropriatechoice of free radical initiator. Free radical initiators whichdecompose quickly but can nevertheless be handled safely are desirable.A good method of testing the decomposition rate of a free radicalinitiator in the high-pressure process is to record the temperatureprofile. For this purpose, the temperature profile is recorded over thelength of the reactor in a polymerization in a high-pressure tubereactor. Immediately after the first introduction of the initiator, thetemperature rises due to the polymerization reaction enthalpy liberatedand then drops again. At the temperature minimum T_(min), initiator isagain introduced and the temperature once more rises steeply and thendrops again. At the next temperature minimum, initiator is again meteredin. The greater the temperature difference between temperature maximumand minimum, the higher the conversion. A critical indication of thecomplete reaction of a peroxide is the cooling curve which is steeperwhen complete decomposition occurs than in cases in which part of theperoxide remains in the reaction mixture even after the actual reactionzone.

In general, a plurality of peroxides of which at least one decomposes ata comparatively low temperature are initially introduced at the startingpoint, i.e. at the beginning of the reactor.

For various reasons it would be desirable to introduce initiator at alarge number of points; however, owing to the high cost of the pumpswhich are necessary at each introduction point, the number ofintroduction points is limited by econimic and engineeringconsiderations.

EP-B 0 813 550 discloses that cyclic peroxo compounds of the formulae P¹to p³ can be used for polymerizing ethylene in the high-pressureprocess.

However, it has been found that the conversion is still too low whenusing the most important conventional free radical initiators. The mostimportant conventional free radical initiators are dibenzoyl peroxide,di-tert-butyl peroxide, tert-butyl perpivalate (“TBPP”) and tert-butylperisononanoate (“TBPIN”). If the conversion is too low, the economicsof the high-pressure process are adversely affected. The conversionswhen using the peroxides of the formulae P¹ to p³ are also too low.

The molecular weight of the product in the high-pressure process can beinfluenced by regulators such as aldehydes, ketones or hydrogen;however, no influence on the covnersion has been found when usingconventional peroxides (GB 1,058,967).

It is an object of the present invention to provide a process by meansof which the conversion in the high-pressure polymerization of ethyleneis increased further.

We have found that this object is achieved by using mixtures ofconventional peroxides comprising from 5 to 95% by weight of commercialcyclic peroxo compounds of the formula I and employing a molarH₂/ethylene ratio of from 1:2 000 to 1:40 000 to increase the conversionfurther in the preparation of polyethylene waxes by the high-pressureprocess, thus making it possible to achieve conversions higher thanthose hitherto customary.

In this formula, the radicals R are identical or different and areselected from among

C₁-C₈-alkyl such as methyl, ethyl, n-propyl, isopropyl, n-butyl,sec-butyl, isobutyl, tert-butyl, n-pentyl, sec-pentyl, isopentyl,n-hexyl, isohexyl, sec-hexyl, n-heptyl, n-octyl; preferably linearC₁-C₆-alkyl such as methyl, ethyl, n-propyl, n-butyl, n-pentyl, n-hexyl,particularly preferably linear C₁-C₄-alkyl such as methyl, ethyl,n-propyl and n-butyl, very particularly preferably ethyl;

C₆-C₁₄-aryl such as phenyl, 1-naphthyl, 2-naphthyl, 1-anthryl,2-anthryl, 9-anthryl, 1-phenanthryl, 2-phenanthryl, 3-phenanthryl,4-phenanthryl and 9-phenanthryl, preferably phenyl, 1-naphthyl and2-naphthyl, particularly preferably phenyl.

The preparation of such trimeric ketone peroxides can be achieved bycondensation of the corresponding ketones with hydrogen peroxide in thepresence of strong mineral acids and is described in the literature (forexample R. Criegee, in Methoden der Organischen Chemie (Houben-Weyl),Vol. 8, p. 46, Georg-Thieme-Verlag, Stuttgart 1952 or EP-A 0 813 550).

The mixtures of the peroxides are made up so that they comprise at leastone peroxide decomposing at high temperature, i.e. it does not decomposeuntil a relatively high temperature is reached, and also at least oneperoxide decomposing at intermediate temperature.

The distinction between peroxides decomposing at high temperature andperoxides decomposing at intermediate temperature is made by means ofthe temperatures at which the half lives t_(½). for the decompositionare 10, 1 or 0.1 hours; it is most usual to report the temperature atwhich the half life is 0.1 hour.

Peroxides decomposing at intermediate temperature have a half life of0.1 hour at temperatures of from 100 to 140° C.

Peroxides decomposing at high temperature have a half life of 0.1 hourat temperatures above 140° C.

There is a wide choice of commercially available peroxides, for examplethe Trigonox® or Perkadox® products from Akzo Nobel. Examples ofcommercially available peroxides decomposing at intermediate temperatureare:

didecanoyl peroxide, 2,5-dimethyl-2,5-di(2-ethylhexanoyl-peroxy)hexane,tert-amyl peroxy-2-ethylhexanoate, dibenzoyl peroxide, tert-butylperoxy-2-ethylhexanoate, tert-butyl peroxydiethylacetate, tert-butylperoxydiethylisobutyrate, 1,4-di(tert-butylperoxycarbo)cyclohexane asisomer mixture, tert-butyl perisononanoate,1,1-di(tert-butylperoxy)-3,3,5-tri-methylcyclohexane,1,1-di(tert-butylperoxy)cyclohexane, methyl isobutyl ketone peroxide,tert-butylperoxy isopropyl carbonate, 2,2-di(tert-butylperoxy)butane andtert-butyl peroxyacetate.

Examples of conventional commercially available peroxides decomposing athigh temperature are:

tert-butyl peroxybenzoate, di-tert-amyl peroxide, dicumyl peroxide, theisomeric di(tert-butylperoxyisopropyl)benzenes,2,5-dimethyl-2,5-di-tert-butylperoxyhexane, tert-butylcumyl peroxide,2,5-dimethyl-2,5-di(tert-butylperoxy)-hex-3-yne, Di-tert-butyl peroxide,1,3-diisopropyl monohydroperoxide, cumene hydroperoxide and tert-butylhydroperoxide.

The trimeric ketone peroxides of the formula I can be classified asperoxides decomposing at high temperature.

The half lives of peroxides are usually determined by a generally usedlaboratory method:

Firstly, a number of ampoules or test tubes containing a dilute solutionhaving a concentration co of less than 0.2 mol/l, preferably less than0.1 mol/l, of the peroxide to be examined are prepared, using an inertsolvent, i.e. one which does not react with peroxides; preference isgiven to benzene, toluene or chlorobenzene.

These ampoules are thermostated at a defined temperature. At definedtime intervals, e.g. 1, 2, 3, 4, 6, 8 hours, an ampoule is taken out,cooled quickly and then analyzed to determine the residual peroxidecontent c_(t). This analysis is preferably carried out titrimetrically.Evaluation is carried out graphically. The relative concentration isplotted logarithmically against the reaction time, so that the half liveat c_(t)/c₀=0.5 can be read off on the ordinate.

To determine the temperature dependence, this measurement is repeated atvarious temperatures.

The mixtures used according to the present invention as free radicalinitiators comprise

from 5 to 95% by weight of one or more trimeric ketone peroxides asperoxides decomposing at high temperature, preferably from 10 to 75% byweight and particularly preferably from 20 to 66% by weight;

from 5 to 95% by weight of one or more conventional peroxides asperoxides decomposing at intermediate temperature, preferably from 25 to90% by weight and particularly preferably from 34 to 80% by weight.

The peroxides, which are extremely shock- and impact-sensitive in thepure state, are advantageously metered as a solution in hydrocarbons,for example using isododecane as solvent. The peroxide mixtures arepresent in the solutions in concentrations of from 5 to 60% by weight,preferably from 15 to 40% by weight.

It is important that the mixture of the peroxides is introduced in thepresence of hydrogen, with a molar H₂/ethylene ratio of from 1:2 000 to1:40 000 being used to increase the conversion further. This proceduresignificantly increases the conversion of ethylene. This is surprisingsince hydrogen has previously been known to perform only amolar-mass-regulating function in the free-radical polymerization ofethylene.

The polymerization of ethylene is usually carried out at pressures offrom 400 to 4 000 bar, preferably from 500 to 5 000 bar and particularlypreferably from 1 000 to 3 500 bar.

The reaction temperature is from 150 to 350° C., preferably from 160 to320° C.

Ethylene is particularly suitable as monomer in the polymerizationprocess of the present invention. It is also possible to preparecopolymers of ethylene, in which case all olefins which can becopolymerized with ethylene by a free radical mechanism are in principlesuitable as comonomers. Preference is given to

1-olefins such as propylene, 1-butene, 1-pentene, 1-hexene, 1-octene and1-decene,

acrylates such as acrylic acid, methyl acrylate, ethyl acrylate, n-butylacrylate, 2-ethylhexyl acrylate or tert-butyl acrylate;

methacrylic acid, methyl methacrylate, ethyl methacrylate, n-butylmethacrylate or tert-butyl methacrylate;

vinyl carboxylates, particularly preferably vinyl acetate, 10-unsaturated dicarboxylic acids, particularly preferably maleic acid,

unsaturated dicarboxylic acid derivatives, particularly preferablymaleic anhydride and alkylimides of maleic acid, e.g. N-methylmaleimide.

Firther suitable molar mass regulators are aliphatic aldehydes, ketones,CH-acid compounds such as mercaptans or alcohols, olefins and alkanesand also mixtures of one or more examples of the various classes ofcompounds. Preference is given to aldehydes and ketones.

The waxes obtainable by the process of the present invention are knownper se. They have molar masses Mw below 20 000 g/mol, preferably below10 000 g/mol and particularly preferably below 7 500 g/mol.

The process of the present invention is illustrated by the examples.

EXAMPLES

The polymerization was carried out in a high-pressure tube autoclave asdescribed in Ullmanns Encyclopädie der technischen Chemie, Volume 19, p.169 and p. 173 ff. (1980). It had the following dimensions: 400 mlength, 32 mm diameter. The experiments were carried out under thefollowing conditions:

Ethylene throughput: 10 metric tons/h

Pressure: 1 900 bar

Propionaldehyde was used as regulator.

Product:

M_(w)=6 300 g/mol

M_(n)=2 100 g/mol

Density=0.918 g/cm³.

Viscosity at 120° C. =1 100 to 1 300 mm2/s

The results are shown in Table 1.

In the examples 1 and 2 according to the present invention, a mixture ofperoxides comprising3,6,9-trimethyl-3,6,9-triethyl-1,2,4,5,7,8-hexaoxanonane (nomenclatureaccording to the Hantzsch-Widmann system) was employed in each case andhydrogen was metered in.

In comparative example C1, the polymerization was carried out without3,6,9-trimethyl-3,6,9-triethyl-1,2,4,5,7,8-hexaoxanonane; in comparativeexample C2, pure3,6,9-trimethyl-3,6,9-triethyl-1,2,4,5,7,8-hexaoxanonane was used asperoxide mixture 2. In comparative example C3, initiation was carriedout using a mixture comprising3,6,9-trimethyl-3,6,9-triethyl-1,2,4,5,7,8-hexaoxanonane but no hydrogenwas added.

TABLE 1 Results of the polymerization experiments H₂ Peroxide PeroxideExperiment [standard H₂/C₂H₄ mixture^(ii) mixture^(iii) Conversion No.m³/h]^(i) [mol/mol] 1 2 T_(start) T_(max) ¹ T_(min) ¹ T_(max) ² T_(min)² T_(max) ³ T_(min) ³ T_(max) ⁴ [%] 1 5 1:25 000 A B 175 285 249 295 241297 249 294 32.1 2 10  1:12 000 A B 174 287 249 301 245 299 252 297 33.0C1 5 1:25 000 A C 174 285 250 293 250 296 260 293 28.1 C2 5 1:25 000 A D173 286 250 276 263 291 269 290 25.3 C3 — 0 A B 174 285 250 294 259 296261 295 29.6 Peroxide mixture A: 40% by weight of tert-butyl perpivalate40% by weight of tert-butyl perisononanoate 20% by weight ofdi-tert-butyl peroxide Peroxide mixture B: 67% by weight of tert-butylperisononanoate 33% by weight of3,6,9-trimethyl-3,6,9-triethyl-1,2,4,5,7,8-hexaoxanonane ^(i)standardNm³/h: standard cubic meters per hour ^(ii)introduced at T_(start)^(iii)introduced at T_(min) ¹ to T_(min) ³. Peroxide mixture C: 67% byweight of tert-butyl perisononanoate 33% by weight of di-tert-butylperoxide Peroxide mixture D: 100% by weight of3,6,9-trimethyl-3,6,9-triethyl-1,2,4,5,7,8-hexaoxanonane The figures in% by weight refer to the calculated content of pure peroxide. For safetyreasons, all peroxides were used as about 15% strength by weightsolutions in isododecane.

We claim:
 1. A process for preparing polyethylene waxes at from 200 to350° C. and pressures in the range from 500 to 4 000 bar using molarmass regulators, wherein a peroxide mixture comprising from 5 to 95% byweight of at least one cyclic peroxide of the formula I,

where the radicals r are identical or different and are selected fromamong alkyl groups and aryl groups, is used as free radical initiatorand a molar H₂/ethylene ratio of from 1:2 000 to 1:40 000 is employed.2. A process as claimed in claim 1, wherein a molar H₂/ethylene ratio offrom 1:5 000 to 1:25 000 is employed.
 3. A process as claimed in claim 1or 2, wherein the radicals R are selected from among linear C₁-C₈-alkylgroups.
 4. A process as claimed in claim 1, wherein all radicals R areethyl.
 5. A process as claimed in claim 1, wherein ethylene iscopolymerized with one or more oliefins which can be copolymerized by afree radical mechanism, preferably selected from among 1-olefins,acrylic acid, acrylic esters, methacrylic acid, methacrylic esters,vinyl carboxylates, unsaturated dicarboxylic acids and derivatives ofunsaturated dicarboxylic acids.